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a Resource 21, 7257 S. Tucson Way, Suite 200, Englewood, CO 80112
b Dep. of Soil Water, and Climate, 1991 Upper Buford Circle, Univ. of Minnesota, St. Paul, MN 55108
c USDA-ARS, Dep. of Crop and Soil Sciences, Washington State Univ., Pullman, WA 99164
Corresponding author (sgupta{at}soils.umn.edu)
Received for publication May 17, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: FWMC, flow-weighted mean concentration • MP, moldboard plow • M, manure • RT, ridge till • U, urea
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In addition to being wet, the soils in the upper Midwest are also cold during early spring, which delays corn planting, shortens the growing season, and thus reduces crop yield (Olsen and Schoeberl, 1970). To overcome problems associated with wet and cold soil conditions during early spring, farmers often moldboard plow in the fall to enhance soil drying and warming the following spring. One consequence of fall moldboard plowing, however, is the lack of surface residue cover during spring when soils are most vulnerable to detachment by spring rains.
Beef, hog, and dairy farming are also an important part of the economy in the area, and thus land application of manure is a common practice. Consequently, additional loading of nutrients and organic matter can occur both in surface runoff and in subsurface drainage. Possible methods to lower nutrient, herbicide, and organic matter loading of the Minnesota River include practices such as ridge tillage, which minimizes soil disturbance while preserving crop residues at the soil surface. Ridge tillage provides substantial residue cover from fall harvest to early spring when ridges are left undisturbed. At planting, surface cover is reduced in the planted row, but overall soil disturbance is minimal. Reestablishment of ridges in late June results in shallow mixing of surface soil, while maintaining cover. Surface residue cover in ridge tillage reduces soil erosion while the raised beds enhance soil warming in early spring (Gupta et al., 1990). One unknown consequence of ridge tillage is the effect of limited soil incorporation of amendments such as fertilizer and manure on nutrient losses in artificial drainage systems.
Extensive literature exists on the effects of tillage, and to some extent, on the effects of manure alone or in combination with different tillage treatments on surface runoff and its quality (Young and Mutchler, 1976; Klausner et al., 1976; Young and Holt, 1977; Wendt and Corey, 1980; Mueller et al., 1984a,b; Converse et al., 1976; Ginting et al., 1998a,b; Hansen et al., 2000). However, most of these studies have been done on steep lands with moldboard and chisel plow systems. There is very limited information on relatively flat (0–2% slope) lands such as those found in the Minnesota River Basin with ridge tillage systems. Furthermore, there was no provision for subsurface drainage in most of these studies. Extensive literature (Gast et al., 1978; Baker and Johnson, 1981; Kanwar et al., 1988; Kladivko et al., 1991; Logan et al., 1993; Randall and Iragavarapu, 1995) also exists on commercial fertilizer effect on subsurface water quality but there is limited data (Randall et al., 2000) on manure effect either alone or in comparison with commercial fertilizer on subsurface water quality. Excellent review articles dealing with surface and subsurface water quality have also been reported (Logan et al., 1980; Fausey et al., 1995; Sharpley et al., 1998; Sims et al., 1998).
The goal of this study was to evaluate tillage and nutrient source interactions on sediment, nitrogen, and phosphorus losses from poorly drained, gently sloping land both in percolate solution via subsurface tile drains and in surface runoff via surface tile inlets. Specific objectives of this experiment were to evaluate the effects of (i) moldboard plowing versus ridge tillage and (ii) solid beef manure versus commercial fertilizer (urea and triple superphosphate) on soil and nutrient losses via surface runoff and subsurface drainage from a major soil in the Minnesota River Basin. The study was conducted immediately after corn planting, when soils were most vulnerable to soil erosion and surface runoff.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Application rates of manure and urea were based on yield goal and the University of Minnesota recommendations (Rehm et al., 1993). Estimated available N for fall manure application was 107, 157, and 157 kg N ha-1 in 1994, 1995, and 1996 (10.5, 17.0, and 22.1 Mg ha-1 of oven-dry beef manure), respectively. Amount of manure applied was based on mineral N and total N analysis. It was assumed that all mineral N (NH+4–N and NO-3–N) and 30% of the organic N in manure was available to corn in the first year (Sutton et al., 1986). Over the 3-yr period, mineral N and total N concentrations of the manure varied from 0.11 to 0.26% and 2.04 to 2.5%, respectively. Phosphorus application due to manure addition equaled 121, 158, and 196 kg P2O5 ha-1 in 1994, 1995, and 1996, respectively.
Spring urea applications in 1995, 1996, and 1997 were 107, 157, and 157 kg N ha-1, respectively. In 1995, a one-time application of triple superphosphate at the rate of 121 kg P2O5 ha-1 was also surface-applied to all urea plots. This application rate was based on soil test P levels and the University of Minnesota recommendations (Rehm et al., 1993). In case of the MP treatment, triple superphosphate was incorporated into the soil to about 5 cm depth during two passes of field cultivation. In the case of the RT treatment, triple superphosphate remained at the soil surface until ridges were rebuilt in late June 1995. In addition to the above manure and fertilizer application rates, a small but equal amount of starter fertilizer was also applied to all plots at planting. Corn was grown each year starting in 1995.
All 16 experimental plots were subjected to simulated corn planting (no corn seed) on 22 Apr. 1997. Starting on 25 April, a 4.9- by 11-m area around the surface inlet in each plot was subjected to simulated rainfall using the rainfall simulator of Hermsmeier et al. (1963). Simulated rainfall was carried out on one plot at a time and therefore it took 5 d to complete the rainfall simulation on all 16 plots. Length of the rain-impacted area coincided with the length of the plot. Before simulated rainfall, soil samples were taken to a depth of 60 cm for antecedent moisture content. Similarly, crop residue cover measurements were also made before rainfall simulation using the line-transect method of Laflen et al. (1981). The residue cover measurements were taken along two diagonal transects in each plot.
Rainfall was applied at an average intensity of 68 mm h-1. The rainfall amount applied to each individual plot varied from 67 to 93 mm, but these differences were not statistically significant (Table 1). Rainfall application averaged over all plots equaled 78 mm, which is an equivalent to rainfall application for 1 h and 10 min. According to the Depth–Duration–Frequency (DDF) curves for the state of Minnesota, natural rainfall amounts for 1-h storm events that occur every 10, 25, and 100 yr are 52, 58, and 72 mm, respectively. Therefore, simulated rainfall intensity of 68 mm h-1 corresponds to a 1-hour-75-year natural rainstorm.
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Water samples from both surface runoff and subsurface tile drainage were analyzed for sediment, NO-3–N, NH+4–N, total P, and soluble P (dissolved molybdate reactive P). Sediments were measured by evaporating 200 mL of water suspension at 105°C. Nitrate N and NH+4–N were analyzed using the conductimetric method of Carlson (1978)(1986). Total P in water suspension was determined by perchloric–nitric acid digestion as described in USEPA standard procedure (USEPA, 1981). Soluble P was measured using the blue molybdate method of Wendt and Corey (1980). Since the water used for simulated rainfall (city water from a local municipal hydrant) contained a high concentration of dissolved salts, gravimetric measurements of sediment in water samples from both surface runoff and subsurface tile drainage were corrected for soluble salts. There was no detectable N and P in the water used for simulated rainfall. Total sediment and nutrient losses from each plot were calculated by multiplying the individual sample concentrations with flow volume and then summing the amounts over the entire period of the simulation.
Analysis of variance (ANOVA) of tillage, nutrient source treatments, or their interactions was performed using SAS (SAS Institute, 1994). The parameters tested were: time to surface runoff via surface inlet, time to percolation via subsurface tile drain, volume of surface runoff, volume of percolate, and the amounts of sediment, NO-3–N, NH+4–N, total N, total P, and soluble P losses in surface runoff, in subsurface tile drainage and the combined flow (surface runoff plus subsurface tile drainage). Since there was a significant tillage by nutrient source interaction on NH+4–N, NO-3–N, total P, and soluble P losses, a pair-wise mean comparison was also performed on these parameters using the method described by Gomez and Gomez (1984)(p. 199–204).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Water Losses
Surface Runoff
Tillage and nutrient source had no significant effect on time to surface runoff via surface inlets, total runoff, or runoff as a percentage of simulated rainfall (Table 1). This lack of tillage and nutrient source effect on surface runoff appears to be due to the lack of differences in surface storage between treatments after secondary tillage and corn planting. Since the ridges were parallel to the direction of slope, there was very little surface storage in the RT treatment. In the MP treatment, two passes of field cultivation in early spring obliterated any surface storage that was present after fall moldboard plowing.
Subsurface Tile Drainage
Moldboard plowing significantly delayed (by 40 min) the start of tile flow compared with RT, while there was no significant difference in the start of tile flow between the manure and urea treatments (Table 1). Early tile flow in the RT treatment appears to be associated with preferential flow, as evidenced by the presence of sediment and NH+4–N in subsurface tile drainage (discussed later). Tillage had a significant effect on peak flow rates. The peak tile flow rates (Fig. 2) were much greater from the RT (RT*M and RT*U) than the MP (MP*M and MP*U) treatments, again suggesting preferential flow in RT compared with the MP treatment. Average peak tile flow rates were 0.64, 0.41, 0.17, and 0.15 cm h-1 for RT*U, RT*M, MP*M, and MP*U, respectively. There was no significant effect of tillage on cumulative tile drainage, thus suggesting nearly equal surface and subsurface storage in the MP and the RT treatments.
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Combined Flow
Tillage and nutrient source had no significant effect on combined (surface runoff plus subsurface tile drainage) water losses (Table 1). Surface runoff was the dominant fraction of the total water losses through artificial drainage. Total water losses as a percentage of simulated rainfall varied from 40% for MP*M to 57% for the RT*U treatment. This shows that surface inlets along with subsurface tiles drain a large volume of water from landscapes in southwestern Minnesota during early spring. This large drainage is desirable for timely crop planting and healthy early crop growth, especially in the fine-textured soils of the region.
Sediment Losses
Surface Runoff
Sediment losses from the MP plots were nearly two times higher than losses from the RT plots (Table 2). Since tillage had no significant effect on surface runoff losses, the increase in sediment losses was mainly due to greater sediment concentration in surface runoff water from the MP compared with the RT treatment. Flow-weighted mean sediment concentration for the MP treatment (3.2 g L-1) was two times more than the concentration from the RT treatment (1.4 g L-1). The higher sediment concentration from the MP plots was due to two major factors: (i) lack of residue cover and (ii) increased surface disturbance. The surface residue cover in the MP plots was 12% compared with 45% in the RT plots. Beside the decrease in soil detachment, higher residue cover between the rows in the RT treatment also decreased the rate of overland flow, reducing its erosive power and trapping sediment from the runoff. After two passes of field cultivation and corn planting, surface soil in the MP treatment was disturbed and had many loose and unconsolidated aggregates (visual observations) that were vulnerable to rainfall detachment and transport. Comparatively, the RT treatment had minimal soil disturbance and few loose or unconsolidated soil aggregates present at the time of simulated rainfall.
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Subsurface Tile Drainage
Contrary to trends in sediment loss in surface runoff, sediment loss through subsurface tile drainage was significantly greater (about five times) from the RT than the MP treatment (Table 2). This increase in sediment loss from the RT treatment was mainly due to preferential flow of water. About 75% of the tile flow samples from the RT treatment had a detectable level of sediment compared with only about 40% of the samples from the MP plots. Subsurface tile flow sediment mostly appeared early in the hydrograph, near the peak flow rate (Fig. 2 and 3). Sediment concentration in subsurface tile drainage from the RT plots was as high as 0.4 g L-1 compared with 0.1 g L-1 from the MP plots. Several hours after the simulated rainfall stopped, sediment in subsurface tile drainage was nondetectable. We hypothesize that the preferential flow in the RT treatment was due to the presence of large continuous pores (macropores) that were not disturbed since harvest the previous fall. These macropores carried some of the runoff and associated sediment to subsurface tile drains. The observation of preferential flow in tile-drained soils has also been reported by Evert et al. (1989). These authors observed the presence of adsorbed tracers, Li+ and Rhodamine WT, reaching the tile line within 25 min after the tracers were applied with irrigation. Also in their study, the timing of peak flow for both adsorbed and nonadsorbed tracers occurred at about the same time after the start of the irrigation. It is also possible that some of the sediment in subsurface tile drainage in our study may have been due to erosion of macropore channels.
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Combined Flow
Most of the sediment losses occurred in surface runoff, varying from 82% for the RT*U treatment to 99% for the MP*M treatment (Table 2). The sediment losses in subsurface tile drainage were as high as 18% of the total sediment losses (RT*U). This identifies preferential flow as an important pathway for sediment loss in the RT treatment.
Tillage treatments had a significant effect on combined sediment losses. Moldboard plowing produced 740 kg ha-1 of sediment compared with 404 kg ha-1 for the RT treatment, a decrease of 45%. Flow-weighted mean concentration (FWMC) for sediment from the MP treatment were >2 g L-1 in comparison with about 0.9 g L-1 for the RT treatment (Table 2).
As with the surface runoff and subsurface tile drainage, manure application had no significant effect on sediment loss in combined flow compared with the urea treatment. There was also no tillage by nutrient source interaction on sediment losses in combined flow.
Nitrogen Losses
Surface Runoff
There was a strong tillage by nutrient source interaction on NH+4–N and NO-3–N losses in surface runoff (Table 3). The RT*U treatment lost more NH+4–N than any other treatment. This was mainly due to a lack of urea mixing into the soil. On the other hand, the RT*M treatment lost more NO-3–N than the RT*U, MP*U, and MP*M treatments. Lower losses of NH+4–N or NO-3–N with moldboard plowing shows that mixing of applied fertilizer by tillage, whether manure or urea, was helpful in reducing mineral N (NH+4–N and NO-3–N) losses in surface runoff. For the ridge tillage treatment, manure application significantly reduced NH+4–N losses compared with the urea treatment, mainly due to slow release of N from applied manure.
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Subsurface Tile Drainage
Tillage had a significant effect on NH+4–N losses via subsurface tile drainage (Table 3). Ridge tillage resulted in 10 times more NH+4–N losses than the moldboard plow treatment, mainly due to poor soil mixing of urea and manure, which in turn, allowed a large amount of dissolved NH+4–N in surface water to percolate through the macropores into tile drains. This explanation is consistent with our earlier observations of preferential flow and greater NH+4–N losses in surface runoff from the RT compared with the MP treatments.
The spring-applied urea treatment resulted in 10 times higher NH+4–N leaching losses than the fall-applied manure treatment. The reduction in NH+4–N losses from the manure treatment was mainly due to slow release of NH+4–N from the organic fraction of the manure compared with very rapid conversion of urea to NH+4–N in the urea plots. Urea hydrolysis depends upon several soil factors including temperature, water content, texture, organic matter content, and depth. For unsaturated flow conditions, the urea hydrolysis rate constant for first-order rate reaction has been shown to vary from 0.016 h-1 (Wagenet et al., 1977) to 0.25 h-1 (Ardakani et al., 1975). These rate constants translate to 11.9 and 2.7 h for 95% of the urea to hydrolyze. Our rainfall simulations did not start for at least 3 d after the application of urea, allowing plenty of time for it to hydrolyze.
Contrary to NH+4–N losses, there was no effect of either tillage or nutrient source treatments on NO-3–N losses via subsurface tile drainage.
Ammonium cations have difficulty passing through soil without being retained by negatively charged organic matter and clay minerals. The appearance of NH+4–N in subsurface flow suggests that the percolating water had little reaction with the soil matrix and was preferentially transported to subsurface tile drains. Since the continuity of preferential pathways (earthworm macropores, cracks) to the soil surface is highly influenced by tillage practices, a significant difference in NH+4–N concentration between the tillage treatments would be expected. On the other hand, NO-3–N anions are easily leached through the soil matrix. This means NO-3–N movement will be controlled by the soil profile characteristics as well as soil surface conditions. These differences in NH+4–N and NO-3–N flow characteristics are illustrated in Fig. 4. Presence of NH+4–N in subsurface tile drainage only occurred at the peak flow period, thus suggesting the occurrence of preferential flow. In contrast, NO-3–N was present both during the preferential and matrix flow periods, although its concentration during matrix flow was relatively constant, thus suggesting a continuous leaching of NO-3–N from the soil profile.
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Combined Flow
There were significant tillage by nutrient source interactions on both NH+4–N and NO-3–N losses in combined (surface runoff and subsurface tile drainage) water flow (Table 3). The RT*U treatment lost at least 11 times more NH+4–N than any other treatment whereas the RT*M treatment lost at least 75% more NO-3–N than any other treatment. The main reason for interactions of tillage with nutrient source on NH+4–N and NO-3–N losses in combined flow is because most of the NH+4–N losses occurred in surface runoff and there was a significant tillage–nutrient source interaction on NH+4–N losses in surface runoff. Although there was no tillage and nutrient source interaction on NO-3–N losses through subsurface tile drainage, the highly significant interaction for NO-3–N losses in surface runoff caused a significant interaction for NO-3–N losses in combined flow. More than 72% of the NO-3–N losses occurred via tile drainage, whereas more than 73% of the NH+4–N losses occurred via surface runoff.
Flow-weighted mean concentrations (FWMC) of NH+4–N in combined flow were 0.12, 0.53, 0.64, and 7.42 mg L-1 for MP*M, MP*U, RT*M, and RT*U treatments, respectively (Table 3). Except for the MP*M treatment, these high concentrations from the other three treatments have the potential to cause harm to humans, whereas the high concentration from the RT*U treatment has the potential to cause harm to both humans and fish (Sharpley et al., 1998). Flow-weighted mean concentrations of NO-3–N were all <2 mg L-1, which is below the 10 mg L-1 water standards of the USEPA (2000).
Phosphorus Losses
Surface Runoff
There was a significant tillage by nutrient source interaction on both total P and soluble P losses in surface runoff (Table 4). Total P losses were greatest from the RT*M treatment followed by the MP*U, MP*M, and RT*U treatments. These treatment rankings are different from the sediment loss rankings because of the differences in soluble P contributions from surface manure and crop residues. Soluble P losses from the RT*M treatment were at least three times more than any other treatment. For the RT*M treatment, 64% of the total P was in the form of soluble P. However, for the MP*U treatment, 91% of the total P was in the form of sediment-associated particulate P. These data suggest that when manure was not well mixed with the soil (RT*M), soluble P from manure as well as from crop residues accounted for most of the total P losses in surface runoff. On the other hand, treatments such as MP*U, which produced the greatest amount of sediment and sediment-bound phosphorus (particulate P), accounted for most of the total P losses. These results suggest that surface-applied manure and crop residue are significant sources of soluble P losses, especially from a water quality perspective. Furthermore, tillage practices that mix manure and crop residue into the soil would significantly reduce soluble P losses. These observations are similar to the findings of Ginting et al. (1998b).
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Soluble P losses in subsurface tile drainage were significantly affected by tillage and nutrient source interactions (Table 4). The RT*M treatment resulted in at least 11 times more soluble P losses than any other treatment. This is consistent with the observation of highest soluble P losses in surface runoff from the RT*M treatment. The above discussion clearly shows that soil mixing of manure is helpful in reducing soluble P losses both in surface runoff and in subsurface tile drainage.
Combined Water Flow
Similar to total P losses in surface runoff, total P losses in combined flow were highest for the RT*M treatment, followed by the MP*U, MP*M, and RT*U treatments (Table 4). Total P losses in surface runoff accounted for about 79% (RT*M) to 100% (MP*U) of the total P losses in combined water flow.
Soluble P losses from the RT*M treatment were 573 g ha-1, much greater than any other treatment (Table 4). Comparatively, soluble P losses from the MP*M treatment were 116 g ha-1 and not significantly different from the RT*U and the MP*U treatments. These data suggest that surface-applied manure in RT would result in significantly higher soluble P losses in combined flow. Of course, a majority of the soluble P losses are in surface runoff. Nevertheless, the treatments that enhance preferential flow (e.g., ridge tillage) could contribute as high as 21% of total P and 24% of soluble P losses in combined flow due to preferential flow pathways.
Flow-weighted mean concentrations of total P were 1.56, 1.64, 1.85, and 0.74 mg L-1 for MP*M, MP*U, RT*M, and RT*U, respectively. These values are much higher than 0.10 mg L-1, the critical concentration for streams (USEPA, 1986). Flow-weighted mean concentration of soluble P was 0.35, 0.14, 1.25, and 0.21 mg L-1 for MP*M, MP*U, RT*M, and RT*U, respectively. Except for the RT*M treatment, the soluble P FWMCs for the other three treatments are below the proposed allowable limit of 1.0 mg L-1 for agricultural runoff (USEPA, 1986).
Overall Evaluation of Water Quality Effects
We characterized the overall effect of the four management treatments on water quality by ranking each treatment from 1 (least effect) to 4 (highest effect) for each of the six parameters (water, sediment, NH+4–N, NO-3–N, total P, and soluble P losses). This ranking was done for both surface and subsurface flow and for the combined flow. Rankings were similar for both losses and FWMC. Therefore, only the loss rankings are presented (Table 5). For each treatment, ranking scores for each loss parameter were summed for an overall score representing the combined effect of the treatment on water quality. This procedure assumes that all six parameters have an equal effect on water quality. Despite this oversimplification, the procedure is helpful in the qualitative ranking of the four treatments.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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